Magnetic fields are widely used in various measuring and imaging technologies, for example, in various kinds of MRI apparatus. In some examples, permanent magnetic fields generated by permanent magnets are employed.
FIG. 1 shows a magnetic arrangement employing C-shaped permanent magnet. The arrangement may be employed in an open MRI apparatus. The magnetic arrangement comprises a C-shaped frame 1 serving as a yoke; upper press plate 2 and lower press plate 21 respectively mounted on the upper and lower ends of an opening of the C-shaped frame, opposing upper pole plate 3 and lower pole plate 31 arranged in the opening of the C-shaped frame; Ring-shaped parts (Rose-ring) 4 and 40 respectively mounted on the opposing surfaces of the upper pole plate 3 and lower pole plate 31; an upper gradient coil and a lower gradient coil (not shown) respectively mounted parallel to the upper pole plate 3 and parallel to lower pole plate 31, and arranged within the respective Rose-rings. An upper magnetic field generating source 5 is arranged between the upper press plate 2 and upper pole plate 3, and a similar lower magnetic field generating source 51 is arranged between the lower press plate 21 and lower pole plate 31. Each magnetic field generating source 5, 51 is composed of a plurality of permanent magnets.
The above magnetic field generating sources 5, 51 are important components of some MRI apparatus. For example, such magnetic field generating devices may be mounted between the press plates 2, 21 and pole plates 3, 31 arranged in the opening of open MRI apparatus, to generate magnetic field strength up to 0.5 Tesla for a whole body MRI apparatus, or up to 1 Tesla for parts of body or animal or industrial MRI apparatus. The homogeneity of the resultant magnetic field is very important to the imaging quality. Generally, the magnetic field is stronger in a central area close to the upper and lower pole plates, 3, 31, but is weaker at the periphery of the pole plates. In order to improve the homogeneity of magnetic field in the space between the upper and lower pole plates, rose-rings 4, 40 are respectively mounted at the periphery of the upper and lower pole plates 3, 31. Because the rose-rings 4, 40 protrude from the surfaces of the upper and lower pole plates 3, 31, the magnetic field strength at the periphery of the pole plates is partially intensified to improve the homogeneity of the overall magnetic field. This improves the homogeneity of magnetic field between the upper and lower pole plates, which is advantageous for imaging purpose. Before a permanent magnet MRI apparatus is dispatched, the magnetic field between the upper and lower pole plates must be adjusted to a homogeneous state, which can be verified with an imaging process.
However, during the course of transportation, installation and operation of the MRI apparatus, the homogeneity of the magnetic field between the upper and lower pole plates 3, 31 will be degraded by environmental changes such as changes of environmental temperature and surrounding magnetic environment. Consequently, it is necessary to re-shim the magnetic field at the operational site to obtain homogeneous magnetic field, and so to allow imaging of the desired quality.
U.S. Pat. No. 4,943,774 discloses a magnetic field adjusting device as shown in FIG. 2. The rose-ring is composed of annular ring 56 and sixteen pieces of movable shim plates 60 mounted thereon. Each shim plate 60 defines two slots 64, and is fixed onto the ring 56 by means of two screws 66 extending through the slots 64. When the magnetic field is to be adjusted (“shimmed”), screws 66 are loosened and the shim plates 60, are moved radially inwards or outwards. The slots 64 slide past screws 66. Once a shim plate is in position, its screws 66 are retightened to hold the shim plate in position. Additionally, to achieve increased homogeneity, one or more of the shim plates can be laminated. The shim plates may comprise a stack of laminations, so that the homogeneity can be achieved by adding or taking out some of the laminations. Limitations of such mechanical homogeneity improving method include the following. A considerable external force is needed to move the shim plates, since the shim plates are strongly attracted to the magnetic field generating source. Friction between the shim plates and the surface means that the shim plate will need to be pushed hard to move at all. Under such a strong force, it is difficult to accurately locate the shim plate in the required position. It is very difficult to accurately control the moving distance or to replace the shimming plug in its original position, as the described screw fasteners 66 do not offer an accurate positioning. It is difficult to measure the displacement imparted to a shim plate, or to be sure that one has replaced a shim plate in its original position. The assembly and disassembly of such structure demands considerable time and man-power, which dramatically increases the cost of installation. The adjusting operation can only be performed on the top layer; and the adjusting range is very limited. The means of disassembling the shim plates or stacked layers of laminations can not realize continuous adjustment. A special disassembling device is needed, which also demands considerable time and man-power, which dramatically increases the cost thereof.